In the conventional and well-known freeze drying process, various types of food such as meats, including beef and poultry, vegetables and fruits are prepared, usually as a cooked product, and immediately thereafter cooled into the frozen state. The frozen food product is subsequently loaded into specially designed vacuum chambers wherein the moisture, which is for the most part in the form of ice crystals in the food, is sublimed from the product, usually under reduced pressure and removed to produce a dried (1-3% moisture) product that is subsequently packed in hermetically sealed containers and stored for use at a later time. The freeze dried food product may be prepared for consumption by the addition of water to effectively restore the moisture and produce a food that very closely approximates the original with respect to appearance, flavor, texture and nutritional quality.
However, during the freeze drying process, the food product retains a major portion of its original volume, even though the weight is greatly reduced due to the removal of water. In addition, the water removal leaves a tissue "skeleton" that tends to be quite fragile and brittle. Any attempts to reduce the volume of the freeze dried product result in destruction of the tissue "skeleton" and the creation of a powdered or fragmented product, which, upon reconstitution with water, produces a "mush" that has no physical resemblance at all to the original food product and is therefore aesthetically undesirable. In addition, the extensive porosity of the tissue skeleton allow permeation by oxidizing gases that may enter the package and thereby degrade the quality of the freeze dried product.
On the other hand, reduction of the volume of the freeze dried product is quite desirable from a shipping and storage standpoint; further, any compaction thereof tends to reduce the vulnerability of the "skeleton" to destruction during handling when transported and stored or during the reconstitution process; and, as is readily apparent, compaction reduces the tissue porosity, thus reducing vulnerability to oxidation deterioration.
Previous attempts have been made to produce a freeze dried product that is capable of being compressed into a smaller volume without destruction of the tissue "skeleton" or interference with subsequent reconstitution into an edible product. In this regard it has been discovered that if almost any food product is freeze dried in a "limited" manner so as to leave behind a moisture content in the range of 5-25%, such a limited freeze dried product may be compressed to perhaps one half to one third or less of its original volume without crumbling or powdering, or appreciable harm to the tissue structures.
The compressed, limited freeze dried product is usually subsequently further dried to reduce the moisture content thereof to the usual 1-3%, after which the compressed freeze dried product is packaged and stored in the customary manner. Such products, upon reconstitution, regain most of their original volume and appearance and for all intents and purposes are comparable with uncompressed freeze dried products.
The production of a satisfactory "limited" freeze dried product by conventional freeze drying techniques is extremely difficult due to the manner in which such conventional processes are carried out. In the conventional freeze drying process, frozen food in the form of relatively small pieces is introduced into a vacuum chamber with facilities for supplying heat to the food pieces to compensate for the endothermic expenditure of thermal energy that takes place during the sublimation of the ice crystals. This heat expenditure, if uncompensated, would result in the reduction of the temperature of the food pieces and thereby diminish the ability to sublime the ice crystals into water vapor. In any event, the food pieces after being sealed into the chamber are supplied with the necessary thermal energy to effectuate sublimation of the ice crystals into vapor, and, at the same time, the atmosphere surrounding the food pieces is reduced in pressure so that sublimation may take place and the water vapor is removed and condensed elsewhere in the system.
At the beginning of the process, the food piece is completely permeated throughout its entire mass with ice crystals which form the major portion by weight of the food. As drying progresses, the ice crystals nearest the food surface sublime, and pass into the vapor phase and are transferred into the drier, very pressure atmosphere surrounding the food particle. As drying continues, the "ice front" retreats further and further into the food piece away from the surface thereof, until eventually the ice crystals in the very center of the food sublime, pass through the surrounding layers of relatively dry food tissue, and from thence into the surrounding dry, low pressure atmosphere.
As will be seen from the above discussion, during the conventional freeze drying process at a time intermediate the completion thereof, the food piece consists of a relatively dry outer layer surrounding an inner frozen core that is as moist as the original food product. It can be seen that if the goal of the freeze drying process is to leave a substantial and uniform moisture content after the freeze drying is completed, the goal is difficult to attain by the fact that the production of a higher moisture content product must of necessity be achieved by stopping the ordinary freeze drying process while a portion of the icecore still remains in the food particle. Such a product, although it has an average moisture content in the desired range, will in fact have a small but very wet and unstable core portion with the outer regions thereof being too dry to behave satisfactorily upon compression. The wet core has never been "freeze dried" while the outer portions have been freeze dried far beyond the desired residual moisture range. Therefore, the approach of partially freeze drying a product by conventional means to achieve a higher moisture end product have been unsuccessful in achieving a desirable product.
In order to avoid the difficulty noted above, it has been proposed to completely freeze dry the food product and then "rewet" the dry product up to the desired moisture level. Such "rewetting" has been attempted by subjecting the dry product to a fine water spray or a steam soaking. However such attempts have met with little success because it is difficult to equally contact all surfaces of the food with the moisture; and, or importantly, the outer food tissues tend to become excessively wet while the core stays dry even though the food is permitted to "equilibrate."